1. Field of the Invention
The invention relates to an air-fuel-ratio control method of an engine and an air-fuel ratio control apparatus for carrying out the same and in particular, relates to an air-fuel ratio control method of an engine in which fuel injection time for realizing a target air fuel ratio is calculated to output a driving signal to an injector, without using data on intake air flow rate, and an air-fuel ratio control system for executing the same method.
2. Description of the Related Art
In the air-fuel ratio control of an engine, intake air flow rate (Qα [kg/h]) is important data for determining basic fuel injection quantity. For example, as shown in Japanese Patent Application Laid-Open No. 57-70934, an air-fuel ratio control method is known in which calculation of an intake air flow rate per one cycle from a map is carried out by using a detected engine revolution number and an intake pipe pressure value as input variables, and detecting of an air-fuel ratio (A/F) from an emission gas component of the engine is executed to perform feedback control of the fuel injection quantity (fuel injection time) to be supplied to the engine.
However, since the above-mentioned detected values have no constant correlation, and it is necessary to store in a memory by using correction quantities for parameters as a map, and many nonlinear elements exist in an engine system, it is practically difficult to obtain an exact intake air flow rate using the map in order to perform an exact air-fuel ratio control.
In order to cope with this, the inventors of the present application already proposed, in Japanese Patent Application Laid-Open No. 2005-140063, a method of estimating the intake air flow rate (Qα [kg/h]) by a predetermined calculation expression on the basis of an engine revolution number (N) and an intake pipe pressure (P [kPa]), without using a map, and of determining a fuel injection time (Ti) on the basis of the intake air flow rate to control an air-fuel ratio. In addition to these data, such a technique is known to use the values of intake air temperature (Tα [° C.]) in order to estimate a more precise intake air flow rate.
Thus, by deriving a predetermined numerical formula from an experimental verification about the physical relationship between the engine revolution number and the intake pipe pressure as described above, it is possible to exactly perform an air-fuel ratio control while sequentially estimating the intake air flow rate. However, the control of the intake air flow rate or an engine revolution number at this time is typically performed by an electronic governor which controls opening and closing of a throttle valve. Here, the air-fuel ratio control is often delayed from the control by the electronic governor by the relationship the fuel injection time (Ti) is obtained by the negative pressure (intake pipe pressure) generated by intake air fed to the downstream of the throttle valve, and a target engine revolution number. In a multipurpose engine which requires low cost and simple construction, the air-fuel ratio control is often performed without using an air fuel ratio sensor.
Therefore, during the transient operation of the engine that the target engine revolution number changes rapidly, there is a case where the air-fuel ratio that is defined as a ratio of an intake air quantity and a fuel amount does not coincide with a target air-fuel ratio. This gives an adverse effect on the response and the emission performance with respect to such a target engine rotational speed that changes in a short time. Thus, the time that is taken to converge into the target engine rotational speed may be delayed, and a situation in which there is a difficulty in maintaining good engine operability might occur.